As is known, circular hosiery knitting machines, particularly single-cylinder circular machines, have knockover sinkers located inside radial slots of an appropriately provided support known as sinker ring, which is monolithically associated with the needle cylinder proximate to its upper end.
The knockover sinkers are staggered with respect to the needles, so that each sinker lies between two contiguous needles, and are actuated, during the rotation of the needle cylinder about its own axis, with a reciprocating motion along a radial direction with respect to the needle cylinder.
More particularly, the knockover sinkers are moved away from the axis of the needle cylinder when the needles, after engaging the thread, start their descent to form new loops of knitting, so that the region of the thread or threads between two contiguous loops rests on the upper portion of the sinkers, which is usually flat and known as knockover plane, while the loops formed previously are knocked over, i.e., released by the corresponding needle. After knocking over the old loops, while the needles start a new upward motion, the sinkers are moved toward the axis of the needle cylinder in order to engage the new loops by means of a hook that lies above the knockover plane, so as to retain and tension the loops against the shank of the needles. This retention against the shank of the needles also has the effect of assuredly opening the latch located proximate to the hook of the needles, while the tensioning, particularly after forming the new loop, shapes the part of said loop produced by the sinker.
The movement of the knockover sinkers along a radial direction with respect to the needle cylinder toward and away from the axis of said needle cylinder is achieved by means of an annular cam, which lies around the axis of the needle cylinder. The movement toward the axis of the needle cylinder, particularly after forming the new loop, is assisted by a complementary cam, and said cam and complementary cam are associated with a sinker cover which lies above the sinker ring and is supported by the supporting structure of the machine.
At least one complementary cam for each feed or drop of the machine is generally provided.
The annular cam and the complementary cams lie around the axis of the needle cylinder and trace a path with portions that move toward the axis of the needle cylinder and portions that move away from it; said path is engaged by a heel of the sinkers, which protrudes upward from the sinker ring, when the needle cylinder is actuated so as to rotate about its own axis with respect to the supporting structure of the machine and therefore with respect to the cover of the sinkers.
In the production of knitted articles it is necessary to be able to advance or delay the movement of the sinkers and to vary the extent of the movement of the sinkers away from the axis of the needle cylinder, in order to be able to obtain high-quality articles by perfectly balancing the part of the loop that is formed by the needle, which can vary in the row, with the part of loop that is formed by the sinker.
To meet these requirements, the cover of the sinkers is supported so that it can rotate about the axis of the needle cylinder, and the complementary cams can move with respect to the sinker cover toward or away from the axis of the needle cylinder; there are also first actuation means, which act on the sinker cover in order to turn it about the axis of the needle cylinder through a preset angle, and second actuation means, which act on command on the complementary cams in order to vary their position with respect to the sinker cover along a radial direction with respect to the axis of the needle cylinder.
In many modern hosiery knitting machines, the first actuation means and the second actuation means are constituted by a first step motor and by a second step motor, which are respectively connected to the sinker cover and to the complementary cams by way of a gear transmission.
The first step motor is usually supported by a fixed structure, while the second step motor is generally mounted on the sinker cover.
However, the gear transmission causes problems and drawbacks.
More particularly, the adoption of a gear transmission, which allows to have an adequately low contrast torque on the motor shaft, requires a relatively large number of components, with consequent production costs.
Moreover, the plays in the meshing of the teeth of the various gears causes instability in the angular position of the sinker cover around the axis of the needle cylinder.
The mounting of the step motor for varying the position of the complementary cams on the sinker cover causes an imbalance and instability of said cover that can cause work defects and poor product quality.